Several investigators have reported an increased risk of breast cancer in women ever having an induced abortion, although some studies have not confirmed these findings. Most of these studies are limited by use of inappropriate reference groups, or not considering potential interactions between induced abortion and major reproductive risk factors for breast cancer, or lack of a consideration of gestational length prior to induced abortion, or inadequate statistical power. The purpose of this proposed nested case control study is to evaluate the possible relationship between breast cancer and induced abortion. The study population will consist of approximately 286,000 women currently enrolled in a randomized trial of breast self-examination (BSE), which has been conducted since 1989 in Shanghai textile workers. Information on demographic characteristics, reproductive history and induced abortion, as well as smoking and alcohol habits, were ascertained in 1989 and 1990 by personal interviews of all women in the cohort. Women are actively followed, and cancer incidence is monitored; and diagnoses of breast cancer are validated by review of pathologic slide. A nested case-control study is proposed. We expect to recruit 500 cases from 1989 to 1994 in the BSE cohort. The same number of controls will be randomly selected from unaffected women in the cohort of the same calendar year of birth and in the same study arm as the cases. Cases and controls will be interviewed in person to obtain detailed information on women's history of induced abortion and other risk factors. If an increased risk in relation to induced abortion is identified, this will provide new knowledge regarding the etiology of breast cancer and the rationale for conducting a detailed study of the endocrinologic response to an induced abortion. More than 1.5 million abortions are performed in the US each year. Our results will thus provide useful information of relevance for women's health and cancer prevention in this country. An important advantage of this study is that it will utilize a very large, accessible, and relatively stable female population currently under study. Another strength is that 53% of these women have had at least one induced abortion and 19% have had 2 or more. Therefore, our study will yield timely and cost-efficient results with high statistical power.